Development of effective therapies for treatment of acute lung injury (ALI) and adult respiratory distress syndrome (ARDS) remains a challenging task. Many experimental models for testing of novel protective agents utilize preventive or concurrent treatment during ALI induction, while post-treatment represents more clinically relevant intervention. Such differences in the timing of drug administration may have dramatic impact on the efficiency of treatment and activation of specific molecular mechanisms directing resolution of ongoing injury in contrast to blocking onset of ALI by drug pretreatment. This proposal will fill this void and explore effects of post-treatment with FDA-approved prostacyclin (PC) analog iloprost in the in vitro and in vivo septic ALI models. Inflammation and increased endothelial cell (EC) permeability play a major role in the pathophysiology of ALI. During the previous cycle of this proposal, we characterized for the first time the molecular mechanisms of PC-mediated protection in aseptic model of ventilator induced lung injury. Our preliminary studies suggest potent protective effects of PC pretreatment against LPS-induced lung inflammation and vascular leak. This proposal will investigate effects of PC post-treatment in cell culture and animal models of septic ALI caused by Gram-positive heat-inactivated Staphylococcus Aureus bacteria (HKSA). We hypothesize that signaling by Rap1 GTPase plays a dual role in PC-induced acceleration of ALI resolution via promotion of EC barrier repair and suppression of inflammatory endothelial activation. Aim-1 will evaluate effects of PC post-treatment and define a role of Rap1 in acceleration of barrier recovery in HKSA challenged EC. Aim-2 will define molecular mechanisms downstream of Rap1 involved in EC barrier recovery. We will study a role of Rap1 effectors KRIT1 and RIAM in enhancement of EC adhesive structures and peripheral cytoskeleton essential for re-establishment of EC barrier. Aim-3 will study a role of Rap1, KRIT1 and RIAM stimulation by PC post-treatment in downregulation of HKSA-induced pulmonary EC activation. Aim-4 will elucidate specific role of Rap1, KRIT1 and Riam in PC-facilitated ALI recovery in vivo using loss of function and gain of function molecular approaches and mouse genetic models. These studies will characterize novel protective mechanisms and identify new protein targets for future therapies aimed at prevention of the pulmonary vascular barrier dysfunction associated with acute lung injury.